Stress distribution and synthetic reinforcing technology for chamber group with soft and fractured surrounding rock

A combination of study methods, including field tests, theoretical analyses and numerical simulations, is employed to investigate the stress distribution around the chamber groups with soft and fractured surrounding rock as well as the synthetic reinforcing techniques in the underground station in Tunliu Coal Mine of Lu’an Group in Shanxi Province. The surrounding rock of the chamber groups belongs to loose, broken and soft rock with medium stress fields. The features of the stress distribution in the surrounding rock are as follows: the secondary stress concentration occurs by overlapping adjacent stress fields at the intersections and corners of chambers, and the peak stress values reach 2-3 times the in-situ stresses. Grouting with high pressure followed by high pretension intensive bolts and cables is an effective synthetic reinforcing form for the soft and broken rock. The grouting with high pressure can fill and glue the fissures to improve the integrity and strengths of the jointed rock mass, meanwhile, it can increase the anchoring capacity of bolts and cables, and be in favor of transferring and diffusing the bolting force in rock mass. The high pretension and intensive bolts and cables can form a compressive reinforcing zone with arch shape. The underground practices indicate that the synthetic reinforcing technique effectively controls the displacement of the surrounding rock mass of the chamber groups, and ensures their long-term stability.